1. Field of the Invention
The present invention relates to a process for the conversion of compounds, in particular but not limited to hydrocarbons, and to apparatus for carrying out the process.
Many chemical processes are used to convert a compound or several compounds into other chemicals or products. The reactant compounds may react immediately on mixing or they may require an energy input to initiate the reaction. The energy required to initiate a reaction is the energy of activation. The energy of activation is typically supplied by a furnace.
Some processes require such a high temperature to supply the energy of activation that the feeds are prone to breaking down into unwanted side products, rather than react to form the desired products. In such cases a catalyst may be used to decrease the energy of activation. This translates into a lower temperature required to overcome the activation energy barrier between the feed and product. Unfortunately, catalysts can often cause undesirable side reactions themselves. Catalysts are also subject to deactivation by reactor feeds, contaminants or products. In addition, the use of a catalyst can significantly increase the complexity of the process being conducted, for example in handling the catalyst and ensuring adequate contact between the components to be reacted and the catalytically active components.
Clearly, uncatalyzed reactions would be preferable if some other form of energy input to overcome the activation energy barrier were available.
One example of such a reaction is the partial oxidation of hydrocarbons to form a mixture of carbon monoxide and hydrogen. The partial oxidation of methane takes place by the following reaction:2 CH4+O2→2 CO+4 H2
The product mixture of this reaction is referred to as synthesis gas or syngas. In the existing art, a substoichiometric mixture of hydrocarbon and oxygen are fed to a burner and combusted at temperatures of 1000° C. to 1500° C. Thermodynamics dictate that carbon dioxide and water will also be produced in the above reaction. Carbon monoxide is thermodynamically favored at higher temperatures. In addition, above 950° C., the following reaction becomes thermodynamically important:2 CO→CO2+C(s)
The degradation of carbon monoxide to carbon dioxide and solid carbon is kinetically slower than the formation of carbon monoxide. Numerous schemes have been devised to thermally quench the synthesis gas product before the degradation reaction can occur.
The partial oxidation reaction can be catalyzed between 800° C. and 1000° C. by noble metal catalysts. Even at this temperature the formation of solid carbon is significant and eventually causes deactivation of the catalyst.
2. Description of Related Art
The problems encountered in the catalytic partial oxidation of hydrocarbons are summarized in U.S. Pat. No. 6,402,989, where it is described that none of the existing processes or catalysts provide a partial oxidation catalyst or process capable of high conversion and high selectivity capable of operation with very low coke formation. Accordingly, as indicated in U.S. Pat. No. 6,402,989, there exists a need for a process and catalyst for the catalytic partial oxidation of hydrocarbons, particularly methane, that has low coke formation, high conversions of methane and high selectivities to CO and H.sub.2, and that is economically feasible at commercial-scale conditions.
The synthesis gas reaction is commercially important for the production of numerous chemicals. More recently it has received increased interest as a means of providing feed for Fischer-Tropsch processes used to convert remote natural gas to low sulfur liquid fuels. These are referred to as gas-to-liquids (GTL) processes. The synthesis gas process is the largest capital and operating expense in a GTL plant.
There is a need for a simple process scheme in which the conversion of compounds, such as hydrocarbons, can be achieved at high efficiencies with a minimum of design and operation complexities.
U.S. Pat. No. 2,958,716 describes a process in which a shock wave is employed to prepare acetylene from hydrocarbons. The process employs a shock wave generated in one of two alternative ways. In a first embodiment, a charge of a combustible mixture is ignited to cause a detonation. The detonation is allowed to pass through a so-called cracking charge consisting of a hydrocarbon to be cracked. The cracking charge comprises a hydrocarbon, such as methane or other lower alkane. Oxygen may be present in the cracking charge, optionally in an amount sufficient to render the cracking charge detonable. U.S. Pat. No. 2,958,716 states that this can lead to increased propagation of the shock wave through the cracking charge. The detonation, if employed, is generated by igniting a combustible mixture of hydrogen and oxygen. The apparatus of U.S. Pat. No. 2,958,716 employs a membrane, such as paper, to separate the detonation charge and the cracking charge from one another prior to the detonation taking place. Some separation of the two charges is also suggested in U.S. Pat. No. 2,958,716 using a layer of inert gas, such as argon. However, the control of the gaseous charges in such an arrangement is particularly difficult. There are no examples of the use of an inert gas separation layer in the specification of U.S. Pat. No. 2,958,716. A further arrangement employs a detonation zone and a cracking zone without any means of separating the detonation and cracking charges.
In a second embodiment of U.S. Pat. No. 2,958,716, a shock wave is generated by the use of a shock tube, in which gas pressure is applied to one side of a rupture disc or membrane. The applied pressure differential across the membrane is sufficient to rupture the membrane, thereby generating a shock wave. The shock wave thus generated is caused to pass through the charge to be cracked. In the examples of this second embodiment, the cracking charge is made up of a lower hydrocarbon, typically methane, admixed with an inert gas, in particular argon and helium, and in some examples oxygen. Hydrogen is used in each example as the pressurised driver gas to create the shock wave. The reliance on the use of a plurality of different gas mixtures and the seeming need to employ inert gases renders the process of U.S. Pat. No. 2,958,716 unattractive for use on a commercial scale.
The specific examples of U.S. Pat. No. 2,958,716 show a range of efficiencies in converting hydrocarbons in the cracking charge using the two embodiments. When the first embodiment is employed, the conversions of the hydrocarbon range from just a few percent to about 19 percent. The second embodiment, using hydrogen as the shock medium, achieved a hydrocarbon conversion to acetylene of almost 25 percent. Using helium as the shock medium and a mixture of methane and chlorine achieved an overall conversion of methane of about 80 percent. The results in U.S. Pat. No. 2,958,716 clearly suggest that the best conversion is achieved using a shock tube arrangement in which a pressurized inert driver gas is used to generate a shock wave to impact the cracking charge.
U.S. Pat. No. 3,192,280 discloses a process in which methane is converted into acetylene. In the process, a combustible mixture of methane and oxygen is fed to the combustion head of a truncated cone reactor. Methane is fed to the reactant head of the reactor. The combustible mixture is ignited, generating a shock wave that resonates within the reactor and converts the methane at the reactant head. A yield of 65% by weight acetylene on the basis of the methane feed is indicated. U.S. Pat. No. 3,192,280 suggests that other hydrocarbons, from C1 to C12 may be used as the feedstock to the reactant head of the reactor. The reactor configuration of U.S. Pat. No. 3,192,280 is complex, requiring a sophisticated arrangement of feed lines and nozzles in both the combustion head and the reactant head of the reactor.
There is a need for a simple process capable of converting a reactive feedstock, that is simple to operate, requires only very simple apparatus and is easily adapted to a variety of scales of throughput and operation.